Concrete mixing trucks are an integral component of construction. Concrete mixing trucks are available in a variety of sizes, and many can carry 10 cubic yards or more of concrete. They are capable of delivering concrete to job sites many miles away. However, the deliverance range is limited by numerous physical variables associated with the perishability of the contents, namely, the concrete.
Concrete is a composite building material. It is unique in that it can be delivered to the construction site in a plastic state and formed to a desirable shape. The most common type of concrete is Portland cement concrete. The two major components of Portland cement concrete are cement paste and inert materials. The inert materials are normally comprised of fine (generally less than 6.4 mm/0.025 in.) and coarse (generally greater than 6.4 mm/0.25 in.) mineral aggregates. Sand is the most common fine aggregate. Gravel, crushed stone, and slag are the most common coarse aggregates.
Other components of concrete include Portland cement, water, and a small amount of air. Portland cement is produced by mixing ground limestone, clay or shale, sand and iron ore. The mixture is heated in a rotary cement kiln. The heating process causes the materials to break down and recombine into new compounds that react with water in a crystallization process called hydration.
Water is the essential component that chemically reacts with the cement, in the hydration reaction. When water is supplied to the concrete, it also provides the initial plasticity necessary to allow the concrete to be poured into forms.
The hydration and setting of concrete is known as curing. Concrete cures in several stages. This allows it to be transported in concrete mixing trucks to construction sites and to be delivered in a condition ready for pouring. Once the concrete is mixed with water, the cement begins a slow cure and the mix hardens. Depending on the exact mixture and additives that may be present, within a day and a half, most of the hydration process is complete, but the cement will continue to cure as long as water and unhydrated compounds are present. The entire process can actually take years.
While the final hardening of concrete can take years, concrete begins to harden soon after mixing. Depending upon the amount of water used, the exact composition of the concrete and ambient weather conditions, concrete will lose its plasticity within a few hours of being mixed, making it unworkable within job site forms.
Workability is the ability of a fresh (plastic) concrete mix to fill the form/mold properly with the desired work (force or vibration) without reducing the concrete's strength. Workability depends on water content, additives, aggregate (shape and size distribution) and age (level of hydration). The level of hydration is very susceptible to environmental factors. In particular, moisture and temperature are critical variables in the curing of concrete.
When the concrete mixing truck arrives at the job site, workability is normally tested by slump measurement. Concrete slump is a simplistic measure of fresh (plastic) concrete's workability.
Concrete transport trucks are designed to transport ready-mix concrete from the concrete plant to the construction site. Rotating drums are employed to prevent premature setting of the concrete, proper mixing of the cement, aggregate, and water, and to provide a mechanism for removing the concrete from the drum. The interior of the drum on a cement truck is fitted with a spiral blade. In one rotational direction, the cement is pushed deeper into the drum while being mixed. This is the direction the drum is rotated while the concrete is being transported to the building site.
In properly mixed concrete, the cementing medium of concrete and water surround the aggregate particles, and as the cement paste hardens, it binds the aggregate into a solid mass. To insure proper mixing, standards have been developed pertaining to the rotation of concrete mixing drums. For example, drums are typically rotated at 17 revolutions per minute (rpm) for approximately 5 minutes (approximately 70 rotations) before leaving the concrete plant. While on the road, the drum will be rotated at approximately 2.5 rpm for the duration of the trip. This further mixes the contents and prevents early hardening of the concrete.
When the concrete transport truck reaches its destination, the rotation of the drum is reversed. When the drum is rotated in the opposite direction, the Archimedes screw-type arrangement forces the concrete out of the drum, and optionally onto slides to guide the viscous concrete, or to a concrete pumping unit.
Concrete must be poured into its final form while in a plastic state, before hardening occurs. If allowed to cure excessively, the concrete will lack adequate workability to be poured into the forms. If allowed to harden, the concrete will be extremely difficult to remove from the interior and exterior surfaces of the concrete mixing truck. Typically, a caustic compound, such as a strong acid, must be used to clean the concrete trucks.
One of the most determinative factors in preserving the workability of concrete in a plastic state prior to pouring is temperature. Depending on the climatic conditions of the region and the season, the cure rate for the concrete will vary dramatically. Additionally, the act of transporting the concrete exposes the concrete mixture to substantial heat transfer mechanisms acting on the concrete mixing truck.
As examples of the heat transfer, conductive heat transfer occurs as a result of the direct contact between the concrete and the interior wall of the drum and the spiral mixing blade. Convective heat transfer occurs between the drum and ambient air as the concrete transport truck travels down the road at freeway speed. Convective heat transfer may be increased by engagement of seasonal winds. Radiant heating of the drum occurs when the summer sun shines on the rotating drum.
In addition to normal influx or loss of heat resulting from weather conditions, concrete curing is an exothermic reaction. Depending on the specific mix of concrete, the heat of reaction can contribute significantly to thermal problems. To a lesser degree, frictional heating occurs on the interior of the drum as the concrete is mixed against the spiral blade on the interior of the drum.
The heat transfer rate is accelerated by the need to rotate the drum, exposing the mixing concrete across the large interior surface of the rotating drum. These heat transfer rates can be as high as 12 BTU/hr-ft2° F. or greater.
As a result, during extended transportation to a construction site, the temperature of the concrete during summer months can accelerate the curing rate such that the concrete is unworkable and/or unusable by the time the concrete mixing truck reaches the construction site. Similarly, the temperature of the concrete during winter months can decelerate the curing rate such that the concrete is unworkable, and/or unprepared for pouring by the time the concrete mixing truck reaches the construction site.
Thermal variations in transported concrete thus add significantly to construction costs, risks, and waste. Delays in construction are a common result. Over the years, numerous steps have been taken by contractors to compensate for the problems associated with climatic temperature variations and heat transfer associated with the transport of viscous concrete. Each of these compensating responses has limitations and disadvantages.
Chemical additives called admixtures are often used to accelerate or retard the hydration rate of the concrete in response to climatic conditions. In particular, a set-retarding admixture may be used to modify setting time in hot weather. An accelerator admixture may be used to modify setting time in cold weather. Plasticizers can also be employed to increase the workability of the concrete. However, the use of admixtures is costly and adds complexity to the process mixing and management of the concrete. Additionally, certain admixtures can undesirably alter the performance characteristics of the finished concrete product.
In addition to the use of admixtures, additional measures are taken when mixing concrete during seasonal temperature extremes. For example, during winter months, preheated water (approximately 180° F.) may be mixed with the concrete in an effort to normalize the temperature of the concrete mixture and accelerate hydration. During summer months, chilled water (near 32° F.) may be mixed with the concrete to normalize the temperature of the concrete mixture and retard hydration. However, utilizing preheated water or chilled water increases the production costs of the concrete. Also, even when steps are taken to manage the initial temperature of the components of the concrete, the mixture within the drum of the concrete transport truck remains subject to rapid heat transfer through the drum resulting from exposure to the ambient weather conditions.
Some other solutions posed are thermal insulation of the concrete drum. In particular, U.S. Pat. No. 6,264,361 to Kelley (“Kelley”), which is hereby incorporated by reference for all purposes, employs a cover for a drum of a concrete truck. Kelley utilizes a pair of thermal blankets over a foam rubber insulation layer to cover the entire drum, using a pair of fasteners to secure the blanket over the drum. Kelley's design, however, can be cumbersome to employ because the disclosed configuration cannot be efficiently manufactured or installed.
Another disadvantage is that the zippers or other fasteners are employed along curved seams. The disclosed fasteners are difficult to secure and may not be suitable for a variety of drum configurations. Another disadvantage is that the zippers or other fasteners are exposed, presenting a safety hazard as the drum rotates. Another disadvantage is that exposure of the zippers or other fasteners to the elements, including concrete, renders them inoperable.
Moreover, Kelley does not consider a number of the existing physical constraints, nor does it provide an optimized solution to several design variables. Considerations such as cost, construction, insulating factor, safety, retention, surface adhesion to concrete, durability, weight, clearances, seasonal assembly and removal, and wind resistance must be taken into account. These considerations, therefore, create a need for an optimized design for an insulating system for a concrete mixing truck.
Additionally, there are other difficulties in manufacturing blankets for concrete trucks, namely difficulty in sewing due to the size. The materials that comprise the concrete truck blankets are oftentimes heavy and cumbersome as well as large in size. Because of these factors, sewing these large pieces of material is extremely difficult, and the inaccuracy of the final product decreases safety and utility. Still another disadvantage is that manufacturing an insulating system as disclosed by Kelley results in the generation of substantial material waste.
Therefore, there is a need for a method and/or apparatus that addresses at least some of the problems associated with conventional insulating blankets.